This application claims the priority of German patent document 196 46 643.1, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system for injecting a nitrogen-oxide-reducing agent into an exhaust stream, especially for an internal combustion engine and an injection element for controlled injection of reducing agent into the exhaust stream.
In combustion processes with a surplus of air, catalytic reduction of the nitrogen oxides that are emitted is not possible without an additional reducing agent. As a remedy, so-called selective catalytic reduction is known from power plant design, in which a reducing agent, for example ammonia or water-dissolved urea, is injected into the exhaust stream upstream from a catalyst. The injected ammonia and/or the ammonia that is released during the pyrolysis of urea reduces the nitrogen oxides in the catalyst to form molecular nitrogen and water. Therefore, a selective catalytic nitrogen oxide reduction for internal combustion engines has also already been proposed, with a suitable reducing agent being injected by conventional injection technology into the combustion chamber or the adjoining exhaust line upstream from an exhaust catalyst. In addition, chemical and physical conditions can be created in the exhaust stream of an internal combustion engine that permit reduction of the nitrogen oxides contained therein, even without a catalyst. This method is termed selective non-catalytic reduction and following conventional injection of the reducing agent, requires that it first be evaporated, increasing the required residence time. It is therefore primarily suitable for engines operating at low rpm.
A system for injecting ammonia into the exhaust stream of a Diesel engine is known from Offenlegungsschrift EP 0 381 236 A1. In that document, a pressurized storage container is provided for the ammonia used as the reducing agent, together with a corresponding supply line and a conventional injection nozzle located upstream from a catalyst for injecting the ammonia into the exhaust stream. The moisture content and temperature of the intake air, fuel consumption, power, and exhaust temperature of the Diesel engine are determined by sensors. Depending on these input values, a control unit controls the quantity of ammonia injected into the exhaust stream by a control valve.
Patent DE 38 21 832 C1 describes an exhaust system for a piston engine with an exhaust turbocharger, said engine being designed for selective non-catalytic nitrogen oxide reduction. An injection element is located upstream of the exhaust turbocharger at a junction of various branches of the exhaust system, said element incorporating a plurality of nozzle openings. By means of a delivery pump, ammonia is added under pressure through an atomizer nozzle to a mixing chamber located upstream of the nozzle openings and mixed there with a carrier gas. The ammonia-carrier gas mixture is then sprayed through the nozzle openings into the exhaust stream under pressure, said pressure being generated by compressors that supply the carrier gas as well as compressed air. The nozzle can be located so that it is axially displaceable in the vicinity of the junction.
Offenlegungsschrift JP 3-206314 (A) teaches an injection device that serves to inject gaseous ammonia into the exhaust stream of a Diesel engine and is located upstream from an exhaust catalyst. A supply tank, with the aid of a compressed gas tank, feeds aqueous ammonia solution into heating tubes introduced transversely into an exhaust line, said tubes terminating in an evaporation-expansion chamber. The evaporation-expansion chamber located in the exhaust line is supplied with evaporated ammonia solution by the heating tubes. The heated gaseous ammonia passes from the evaporation-expansion chamber through a porous nozzle into the exhaust stream.
A conventional atomizer nozzle that can be used for injecting a reaction medium, urea for example, into an exhaust stream is described in Offenlegungsschrift EP 0 586 913 A2. A mixing chamber located outside the atomizer nozzle is supplied with gas under pressure and a liquid reaction medium. The reaction medium-gas mixture produced in the mixing chamber passes through a mixing line into a nozzle prechamber of the atomizer nozzle, provided with a screen. The mixture is accelerated through an intermediate nozzle and enters the main chamber of the atomizer nozzle where further mixing occurs. The reaction medium-gas mixture passes from the main chamber through nozzle openings into the exhaust stream upstream from a catalyst.
Micromechanical spray jet elements that have a plurality of fine nozzle openings through which a supplied fluid is delivered in the form of fine streams by means of a pulsed pressure produced locally in the vicinity of the nozzle openings are used in ink jet printers. To produce local pressure, electrical heating resistors are used in so-called "bubble jet" printers, while piezoelectrically actuated membranes are used in "piezo jet" printers. By means of the micromechanical spray jet element, ink or printing ink is sprayed onto paper, with the very small nozzle openings producing fine distribution and hence a high spatial resolution. Ink jet printer nozzles of these designs are described for example in Offenlegungsschriften DE 30 12 936 A1 and DE 195 31 740 A1.
The invention has as the technical problem to be solved the provision of a system with which a reducing agent can be added to the exhaust stream at relatively low cost and with comparatively good mixing.
The invention achieves this goal by providing a system with a micromechanical injection element and a reducing agent injection control unit. In this system, the nitrogen-oxide-reducing agent is injected into the exhaust stream of an internal combustion engine through a controllable injection unit that contains a micromechanical spray jet element that uses technology found in ink jet printers, with the reducing agent being sprayed controllably into the exhaust stream with good mixing through a plurality of fine nozzle openings in the form of fine streams by means of locally generated pulsed pressure. The use of micromechanical spray jet elements with local pressure generation limited to the nozzle opening area has a simple system design, without any pressure- carrying lines or separate pressure pumps. A reducing agent injection control unit controls the injection time and/or the volume of reducing agent injected into the exhaust stream of an internal combustion engine, at least as a function of the engine rpm, engine load, crankshaft angle, and/or exhaust temperature. By means of this injection that depends upon the operating parameters, good nitrogen oxide reduction in the exhaust stream is achieved and at the same time overdosage of reducing agent is avoided.
In one embodiment, heating resistance elements are specially provided in the vicinity of the respective nozzle openings as means for local pressure generation, by which elements the reducing agent supplied in liquid form is locally superheated and consequently injected in the form of fine vapor streams into the exhaust stream. Preliminary reactions that have a positive effect on later reduction of nitrogen oxides can be triggered during the injection process by virtue of injection in the vaporized state, and mixing of reducing agent with the exhaust stream is facilitated.
In another embodiment, the means for local pressure generation are designed specially as piezoelectrically operated membranes located in the vicinity of the respective nozzle openings. The use of a piezoelectrically operated spray jet element of this kind permits especially accurate metering of the injected reducing agent.
In another embodiment, the nozzle openings are distributed over a large area along the exhaust flow path. The reducing agent can therefore be added at relatively low cost to the exhaust stream, with comparatively good mixing and distribution.
In another embodiment, provision is made for selective non-catalytic nitrogen oxide reduction in an internal combustion engine such that the nozzle openings terminate in the combustion chamber and/or in the exhaust manifold and/or directly upstream from the exhaust turbine of an exhaust turbocharger, and the injection control unit triggers injection of reducing agent only when the exhaust temperature is higher than a predetermined minimum temperature. The temperature is chosen so that injection of reducing agent is prevented when the exhaust temperature is insufficient for nitrogen oxide reduction.
In another embodiment, provision is made for selective catalytic nitrogen oxide reduction by an exhaust catalyst located in the exhaust flow path such that the nozzle openings terminate in the exhaust flow path upstream of the catalyst and the control unit triggers injection of reducing agent only when the exhaust temperature is higher than a predetermined minimum.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.